The New Testament didn’t exist until the pope declared it as such in the 4th century.
What? Like, as books? The books were gradually written over thousands of years, they weren’t given to us by Catholicism.
Also, I find it strange that you think 2 Thessalonians 2:15 mandates adding to Scripture, as it doesn’t seem like that’s what’s explicitly being encouraged here. Can you explain why you think this passage condones the addition of new rules to what God has given us in the Scriptures? Was what God gave us not good enough, that we have to make stuff up in order for our salvation to be complete?